1. Field of the Invention
The present invention relates to an actuator containing a conductive polymer that acts as an actuator element.
2. Description of the Related Art
A conductive polymer doped with a dopant (a dopant-containing conductive polymer) such as a polypyrrole doped with a dopant can be expanded and contracted when electrochemically oxidized and reduced. In view of the electrolytic expansion/contraction properties, actuators using a thin film of the dopant-containing conductive polymer as an actuator element have been studied as described in Japanese Laid-Open Patent Publication No. 2004-162035. For example, when the dopant-containing conductive polymer (the actuator element) is immersed in an electrolytic solution and a voltage is applied thereto, ions in the electrolytic solution are introduced into the actuator element, whereby the actuator element is expanded. When a voltage is applied in the reverse direction, the introduced ions are released from the actuator element, whereby the actuator element is contracted.
This type of actuator element (in other words, the thin film of the dopant-containing conductive polymer) can be produced by adding the dopant and a monomer to a solvent and by applying an appropriate voltage through a pair of electrodes immersed in the solvent. In this case, an electrolytic oxidative polymerization occurs, so that the thin film of the dopant-containing conductive polymer is deposited on a surface of the positive electrode.
It is obviously preferred that the actuator element has a high expansion/contraction ratio. From this viewpoint, in Japanese Laid-Open Patent Publication No. 2004-162035, a trifluoromethanesulfonate ion and/or an anion that contains a plurality of fluorine atoms capable of bonding to a central atom, is used as the dopant while a positive electrode is made of metal, to obtain the dopant-containing conductive polymer with a high expansion/contraction ratio.
In specific examples (Examples 1 to 17) of Japanese Laid-Open Patent Publication No. 2004-162035, a polypyrrole doped with a dopant of a tetrafluoroborate ion or a trifluorosulfonate ion is used as an actuator element, and the actuator element is immersed in a 15%-by-weight aqueous solution of sodium hexafluorophosphate to produce an actuator. Paragraphs [0074] and [0076] of Japanese Laid-Open Patent Publication No. 2004-162035 reports that, while a polypyrrole doped with a benzenesulfonate ion or a p-toluenesulfonate ion had an expansion/contraction ratio of less than 3%, the actuator elements of Examples 1 to 17 each had an expansion/contraction ratio of 3% to 5% under 1-cycle potential application.
As a result of intense research, the inventors of the present application have found that, though the expansion/contraction ratios of the actuator elements of Examples 1 to 17 according to Japanese Laid-Open Patent Publication No. 2004-162035 are 3% or more in the first cycle, the ratios are rapidly lowered in the second cycle or later. Thus, the actuator element of Japanese Laid-Open Patent Publication No. 2004-162035 is poor in durability, and thereby is significantly disadvantageous in difficulty of maintaining the expansion/contraction ratio.